1. Field of the Invention
This invention relates to fuel ignition systems of the intermittent pilot type, and more particularly, to control arrangements for use in such systems which provide fail-safe operation of the fuel valves of the systems.
2. Description of the Prior Art
In intermittent pilot type fuel ignition systems, a pilot valve is operated at the start of an operating cycle to supply fuel to a pilot outlet for ignition to provide a pilot flame. A pilot flame sensor, which typically includes an electronic flame sensing circuit, detects the pilot flame and only then effects energization of the main valve, generally by operating a "flame relay", to supply fuel to a main burner which is ignited by the pilot flame.
In order to prevent unintended actuation of the main valve under fault conditions of the flame sensing circuitry or for a welded contact failure of the flame relay, relay checking arrangements have been incorporated into intermittent pilot ignition systems to test the integrity of the flame sensing circuit and the flame relay before the main valve is operated. In such arrangements, a control or "checking" relay is energized at the start of an operating cycle over a circuit path which includes normally closed contacts of the flame relay. The checking relay operates to close normally open contacts which are connected in the energizing path for the main valve, permitting the main valve to be energized when the flame relay operates. If for any reason the flame relay is operated at the start of an operating cycle so that its normally closed contacts are open, then the checking relay cannot operate thereby preventing energization of the main valve.
In some relay checking arrangements, the pilot valve is also energized under the control of the checking relay. Such arrangements permit both pilot and main valves to be maintained deenergized for a fault which prevents normal system operation.
A further improvement in control arrangements has been the addition of a timing device to time the interval for which the pilot valve is operated at the start of a heating cycle. One such system, disclosed in my U.S. Patent application Ser. No. 790,408 filed on Apr. 25, 1977, and now U.S. Pat. NO. 4,178,149, employs a mechanical warp switch timer which enables the pilot valve to be energized only for a pretimed duration at the start of an operating cycle. The warp switch timer defines the trial for ignition interval and deenergizes the pilot valve at the end of such interval unless a pilot flame is established. Under normal operating conditions, a pilot flame is established before the warp switch times out, and the flame relay operates to override the warp switch timer, permitting the pilot valve to remain energized. For a flameout following a successful ignition cycle, the timer is reenabled under the control of the flame relay to define a further timing interval and to effect deenergization of the pilot valve if a flame is not provided before the timer times out.
In this arrangement, the warp switch timer limits the time for which the pilot valve can remain energized in the absence of a flame. However, the operation of the warp switch timer to provide its timeout signal is predicated on the ability of the flame sensing circuit and flame relay to indicate loss of flame. Also, in known arrangements which employ mechanical warp switch timers to define the trial for ignition interval, certain fault conditions of the warp switch timer may result in lengthening the trial for ignition interval, an undesireable condition.